4 Answers
4

Picasa stores face tag information in a database file and a .picasa.ini file. Both can be found in the directory where the images reside. To find where your images are, right click one and select "locate on disk".

I do not think the database is stored next to the images; rather in the system's local app data directory. At least on Windows.
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LeftiumOct 1 '09 at 1:40

The faces are indeed there (I thought the Picasa was no longer using .Picasa.ini). The only problem is that the names are crazy linked to some address book in Picasa Web, so you are to reenter the names in the other computer (no big deal, but, Google: using a simple name would be better)
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Eduardo MolteniOct 1 '09 at 13:02

The face tag information is stored in .picasa.ini files.
Although the tags are actually references to contacts that are stored in user's directory.
On my PC (Windows Vista) the contacts are stored in %LocalAppData%\Google\Picasa2\contacts\contacts.xml file. The contacts can be local (sync_enabled="0" in the XML file), so there is no need to sync with the web server.

So, if you synchronize your contacts.xml between your PCs the face tag information should be synchronized as well. The question is how to do it.
This howto describes how to share picasa data between multiple accounts on same PC. Ideally picasa local data should be put on the same server where you store your pictures and then %LocalAppData%\Google\Picasa2* directories are to be linked to the remote directories. Unfortunately I don't know a way of creating links to remote shares. NTFS junctions obviously don't work in this case. I'm not a Windows expert though.
If you can't link to a remote share, you will have to synchronize your picasa folders in some other way.

Update: just found a "WinXP - Map network location to local folder" (can't post a link because I don't have enough reputation points) thread on serverfault.com. Conclusion there is that you can't map a shared folder to a local folder (not a drive letter).

Ah, excellent work with the contacts.xml there. It looks like that plus the .picasa.ini files should do the trick for restoring my faces after an XP reinstall... although it looks like Picasa will want to rescan all the photos. Oh well, that can be done over a night, I think.
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NathanielJan 2 '10 at 22:50

This is the response from an actual Google Picasa team member, according to www.digitalhomethoughts.com:

A: There are a few different ways to
transfer your face tags:

You can perform a Backup of your photos and restore them on your new
computer. This process will preserve
your face tags. ("Tools" > "Backup
Pictures")

You can copy your files and the .picasa.ini file from your hard drive
to the new destination

Upload your face tag albums to Picasa Web Albums and download the
album to the new destination

*Important note on option 3: In order for your contact information to be
preserved you'll need to make sure
that your contacts associated with
your face tags are marked as "Online
Contacts." This means that the person
is synchronized with Google Contacts.
After copying the photos to the
destination computer, make sure you
are logged in to your Google user
account so that Picasa can retrieve
your Google Contacts and update your
face tags with the corresponding
information. See reference section for
more info on syncing contacts."

Also, the Picasa team is supposedly working on a more portable implementation of face tags.

(1) and (2) didn't work for me. Looking at picasa.ini, I'm not sure how it could. It didn't even remember the associations I made between related faces. (3) worked but it created new people instead of using the already created ones. Why can't they just put it in EXIF/IPTC?
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hyperslugOct 1 '09 at 2:36

@hyperslug: When you tried (1) and (2), did you ensure you were logged into Picasa web albums from the client and syncing of contact info was enabled on all computers? I believe the ini file only holds references to the tagged people, and maybe specially encoded rects.
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LeftiumOct 1 '09 at 8:17